We have obtained labelled DNA preparations from human, cat, rat, mouse, rhesus monkey, chick and toad (Xenopus laevis) cells. Unique sequences have been obtained from each of these with specific activities from 50-2000,000 cpm/micron g DNA. Optimal conditions for obtaining such high specific activity DNA have been worked out. In addition, studies have been done to optimize recovery of hybridizable RNA sequences from crude homogenates. We have also found ways to double the incorporation of (H3) uridine into RNA in cultured cells. We have also determined the optimal conditions for analysis of the hybridization reaction with Sl nuclease. At present we are analyzing RNA diversity in brains of rats raised in environments of different experimental complexity. These studies will utilize brains of animals so reared by Bennett, et. al., as well as animals raised in our own laboratory. The plan of action is implicit in the above discussion. Small differences in sequence diversity may be amplified for further study by modifications of the hybridization techniques. If specific RNA sequences are uncovered, they may be further evaluated by use of complementary DNA and other techniques already in the literature. At present we have unique sequences of all of the above labelled DNA's, have collected many specimens for RNA extraction, and have performed the first hybridization experiments.